1. Field of the Invention
This invention relates to processes for producing highly textured, biaxially aligned superconducting ceramics in composite conductor forms from metallic precursors, and to the resultant articles.
2. Background of the Invention
Since their discovery less than a decade ago, the superconducting ceramics have attracted wide interest, but many practical applications hinge on their ability to carry currents (below critical temperature, field and current values which are characteristic of each material) almost without resistive losses. The potential of these materials to support high current densities (J.sub.c 's) has been demonstrated by the high values achieved with epitaxially grown thin films in small sample sizes. However, in the bulk forms desired for most electrical and magnetic applications, reported J.sub.c 's are much lower because of losses at the boundaries between the individual grains of polycrystalline material. Lower J.sub.c 's, for example, are typical of composites formed, for example, by the well-known powder-in-tube (PIT) process. Nonetheless, composite conductors produced using PIT techniques are considered the most promising choice for bulk electrical and magnetic applications, as they offer better mechanical properties than the brittle superconducting oxides alone can provide and a scalable, cost-effective method of manufacturing long lengths of wires and tapes.
PIT conductors are composites including one or more filaments of superconducting material in intimate contact with a metal matrix. Several variations have been developed, depending on the type of precursor material used. These are described, for example, in A. Otto, L. J. Masur, C. Craven, D. Daly, E. R. Podtburg, and J. Schreiber, "Progress toward a long length metallic precursor process for multifilament Bi-2223 composite superconductors", IEEE Transactions on Appl. Supercon., vol 5, No. 2 (Jan. 1995), pp. 1154-1157 in U.S. Pat. Nos. 4,826,808, and 5,189,009 to Yurek et al., in W. Gao & J. Vander Sande, Superconducting Science and Technology, Vol. 5, pp. 318-326, 1992, in C. H. Rosner, M. S. Walker, P. Haldar, and L. R. Motowido, "Status of superconducting superconductors: Progress in improving transport critical current densities in superconducting Bi-2223 tapes and coils" (presented at conference `Critical Currents in High Tc Superconductors`, Vienna, Austria, April, 1992) and K. Sandhage, G. N. Riley Jr.,. and W. L. Carter, "The oxide-powder-in-tube method for producing high current density BSCCO superconductors", Journal of Metals 43,21 (1991) which teach the use of either a mixture of powders of the oxide components of the superconductor (OPIT) or of a metal alloy powder having the nominal elemental composition corresponding to the cation stoichiometry of the desired superconducting oxide (MPIT), all of which are herein incorporated by reference.
FIG. 2 (prior art) is a schematic diagram illustrating the main steps in manufacturing superconducting composites by the prior art MPIT method. In step 201, the elemental metallic constituents of the desired superconducting ceramic are mechanically alloyed to form a homogeneous alloy powder, as further described in Otto et al, cited above. Repeated mixing, deformation, compression welding and fracture decease the length scale of phase inhomogeneity until a quasi-amorphous and malleable alloy of the elements results. In step 202, the precursor alloy powder is packed into a first malleable metal can, preferably of a noble metal, that is then sealed and longitudinally deformed in step 203, preferably by extrusion, to form a monofilamentary rod of reduced cross-section including one or more filaments of precursor material surrounded by a noble metal matrix. In step 204, cut pieces of the rod are packed into a multirod bundle that is packed into a second malleable metal can and longitudinally deformed in step 205 to form a multifilamentary rod. Steps 204 and 205 are repeated as many times as necessary to form a composite wire or tape with the desired number and size of filaments. The composite is then oxidized, to form intermediate suboxide phases from the precursor alloy, as shown in step 206, and these intermediate phases are reacted in step 209 to form a superconducting oxide which may be either the final, desired superconducting oxide or an intermediate superconducting oxide.
A key requirement for improving the Jc of polycrystalline superconducting oxides is a high degree of crystallographic alignment or texture of the superconducting grains. In conventional PIT processing, as shown in step 210, non-axisymmetric deformation is used to physically align the grains of intermediate or final superconducting oxide phases in the desired direction, namely primarily such that the c-directions of the grains are aligned orthogonally to the desired current direction along the tape axis. This type of uni-axial texturing has been particularly well developed for the PIT fabrication of the micaceous bismuth-strontium-calcium-copper-oxide (BSCCO) 2212 and 2223 superconducting phases because these oxides exhibit a modest amount of plastic deformation via the activation of a c-plane slip system.
It is important to note that in conventional PIT processing, the deformation is applied directly to the superconducting oxide phases, which either possess a single slip system as in the case of the BSCCO superconducting oxides, or no active slip system at all, as is the case with all the rare earth-containing superconducting copper oxides, the thallium-containing superconducting copper oxides and the mercury-containing superconducting copper oxides. Therefore, post elastic strain is accommodated primarily by the breakup of the oxide into particles consisting of individual superconducting grains as well as secondary phase grains, or particles that contain clusters of these phases, and the deformation then induces texture by rotating the aspected plate shaped grains of the superconducting phases such that the large flat surfaces of the grains that are orthogonal to the grain c-directions (i.e., the c-planes) align with the deformation plane. Following this type of deformation texturing, the subsequent processing must re- sinter the oxide particle fragments, as shown in step 211, via a commonly employed reaction sintering heat treatment that converts a deformation textured precursor superconductor phase to the desired superconductor (in the case of BSCCO, from BSCCO 2212 to BSCCO 2223). For some superconducting materials, a final post-sintering heat treatment, shown in step 212, may be performed to optimize the defect chemistry of the superconductor.
In spite of progress however, the utility of BSCCO superconducting phases is in doubt for field generating applications operating above about 50 K. The primary problem with even well uni-axially textured BSCCO superconducting phases attaining via PIT processing is that their Jc's degrade in small magnetic fields. Unlike the BSCCO superconducting phases (2201, 2212 and 2223) alternate superconducting copper oxide phases such as the yttrium-barium-copper-oxide (YBCO) 123, 247, and 124 phases, and the thallium-barium-calcium-copper-oxide 1212 and 1223 phases are inadequately deformation textured in PIT processes. However, suitably textured thin films of these phases do exhibit very high Jc levels in high magnetic fields above 50 K, demonstrating their intrinsic potential for use in field generating applications if long lengths of multifilament conductors with suitably textured oxide forms can be economically manufactured. FIG. 1 (prior art) shows the J.sub.c dependences on applied magnetic field for untextured, bulk melt textured and textured thin film forms of YBCO 123.
Although direct deformation induced bi-axial texture has been reported for the YBCO 124 non-micaceous superconducting phase via the deformation of the oxides in "Bi-axial texture in Ca.sub..1 Y.sub..9 Ba.sub.2 Cu.sub.4 O.sub.8 composite wires made from metallic precursors" L. J. Masur, E. R. Podtburg, C. A. Craven, A. Otto, Z. L. Wang, D. M. Kroeger, J. Y. Coulter and M. P. Maley, Physica C 230 (1994) pp.274-282, the amount of texture was inadequate for attaining non-weak linked Jc in field, and adequately high Jc levels because the bi-axial texture was confined to local regions of each filament,such that regions with high levels of internal biaxial texturing were out of alignment relative to each other. Furthermore, the deformation of the oxide filaments presents the subsequent problem of resintering the fractured oxide particles.
Known techniques for producing epitaxial growth in superconducting ceramics are intrinsically slow and, to date, very expensive, making them unsuitable for the long length conductors needed for most electrical and magnetic applications. It has not heretofore been considered possible to obtain the finely textured substrate layers and thin layers of superconducting material required for epitaxial growth by any low cost, high volume manufacturing method.
Therefore, it is an object of this invention to provide epitaxially grown, highly textured, biaxially aligned superconducting ceramics in forms suitable for long length conductors.
It is also an object of this invention to provide a method for producing these conductors by a low cost process suitable for high volume manufacturing.